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<h2>CHAPTER VIII.</h2> 146. Which are the better conductors of heat, fluids or solids? Generally speaking, solids, especially those of them that are dense in their substance. 147. Why are dense substances the best conductors of heat? Because the heat more readily travels from particle to particle until it pervades the mass. 148. Why are fluids bad conductors of heat? Because of the want of density in their bodies; and because a portion of the imbibed heat always passes off from fluids by evaporation. [Pg 51] <hr class="bible-verse" /> "He casteth forth his ice like morsels: who can stand before his word,"—Psalm cxlvii. <hr class="bible-verse" /> 149. Why are woollen fabrics bad conductors of heat? Because there is a considerable amount of air occupying the spaces of the texture. 150. Is air a good or a bad conductor? Air is a bad conductor, and it chiefly transmits heat, as water does, by convection. 151. Is water a good or a bad conductor? Water is an indifferent conductor, but it is a better conductor than air. 152. Why, when we place our hands in water, which may be of the same temperature as the air, does the water feel some degrees colder? Because water, being a better conductor than air, takes up the warmth of the hand more rapidly. 153. Why, when we take our hands out of water do they feel warmer? Because the air does not abstract the heat of the hand so rapidly as the water did, and the change in the degree of rapidity with which the heat is abstracted produces a sensation of increased warmth. 154. Why do we see blocks of ice wrapped in thick flannel in summer time? Because the flannel, being a non-conductor, prevents the external heat from dissolving the ice. Flannel wrapped around a warm body keeps in its heat; and wrapped around a cold body, prevents heat from passing into it. 155. How do we know that air is not a good conductor of heat? Because, in still air, heat would travel to a given point much more rapidly, and in greater intensity, through even an indifferent solid conductor, than it would through the air. 156. How do we know that water is not a good conductor of heat? [Pg 52] Because in a deep vessel containing ice, and with heat applied at the top, some portion of the water may be made to boil before the ice, which lies a little under the surface, is melted. <hr class="bible-verse" /> "As snow in summer, and as rain in harvest; so honour is not seemly for a fool."—Prov. xxvi. <hr class="bible-verse" /> 157. Why would you apply the heat at the top, in this experiment? Because in heating water it expands and rises. The boiling of water is caused by the heated water ascending from the bottom, and the colder water descending to occupy its place. If the heat were not applied at the top, it would be distributed quickly by convection, but not by conduction. 158. Why are bottles of hot water, used as feet-warmers, wrapped in flannel? Because the flannel, being a bad conductor, allows the heat to pass only gently from the bottle, and preserves the warmth for a much longer time. 159. Why are hot rolls sent out by the bakers, wrapped up in flannel? Because the flannel, being a bad conductor, does not carry off rapidly the heat of the rolls. 160. Why is it said that snow keeps the earth warm? Because snow is a bad conductor, and prevents the frosty air from depriving the earth of its warmth. 161. Why are snow huts which the Esquimaux build found to be warm? Because snow, being a bad conductor, keeps in the internal heat of the dwelling, and prevents the cold outer air from taking away its warmth. 162. Why is snow, being composed of congealed water (and water being a better conductor than air), so good a non-conductor? Because in the process of congealation it is frozen into crystalline forms, which, being collected into a mass, form a woolly body, thus [Pg 53] proving the truthfulness of the Bible simile, which says, God "giveth snow like wool." <hr class="bible-verse" /> "He giveth snow like wool: he scattereth the hoar frost like ashes."—Psalm cxlvii. <hr class="bible-verse" /> <div class="figcenter"> <ANTIMG src="images/i-053.jpg" width-obs="500" height-obs="354" alt="" /> <div class="caption">FIG. 1.—CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE.</div> </div> 163. Why does it frequently feel warmer after a frost has set in? Because, in the act of congealation a great deal of heat is given out, and taken up by the air, and thus the severity of the cold is in some degree moderated. 164. Why is it frequently colder when a thaw takes place? Because, in the process of thawing, a certain amount of heat is withdrawn from the air, and enters the thawed ice. 165. What benefit results from these provisions of Nature? They moderate both the severity of frosts, and the rapidity of thaws, which, in changeable climates, would be seriously detrimental to life, and to vegetation. 166. Why are furs and woollens worn in the winter? Because, being non-conductors, they prevent the warmth of the body from being taken up by the cold air. 167. Why are the skins of animals usually covered with fur, hair, wool, or feathers? [Pg 54] Because their coverings, being non-conductors of heat, preserve the warmth of the bodies of the animals. <hr class="bible-verse" /> "He sendeth out his word, and melteth them: he causeth his wind to blow, and the waters to flow."—Psalm cxlvii. <hr class="bible-verse" /> 168. How is the greater warmth of animals provided for in the winter? It is observed that, as winter approaches, there comes a short woolly or downy growth, which, adding to the non-conducting property of their coats, confines their animal warmth. In small birds during winter, let the external colour of the feathers be what it may, there will be found a kind of black down next their bodies. Black is the warmest colour, and the purpose here is to keep in the heat, arising from the respiration of the animal. 169. How is warmth provided for in animals that have no such coats? They are furnished with a layer of fat, which lies underneath the skin. Fat consists chiefly of carbon, and is a non-conductor. 170. Why are summer breezes said to be cool? Because, as they pass over the heated surface of the body, they bear away a part of its heat. 171. Why is a still summer air said to be sultry? Because, being heated by the sun's rays, and being a bad conductor, it does not relieve the body by carrying off its heat. 172. Why does fanning the face make it feel cooler? Because, by inducing currents of air to pass over the face, a part of the excessive heat is taken up and carried away. 173. Why does perspiration cool the body? Because it takes up a part of the heat, and, evaporating, carries it into the air. 174. Why does blowing upon hot tea cool it? Because it directs currents of air over the surface of the tea, and these currents take up a part of the heat and bear it away. 175. Why does air in motion feel cooler than air that is still? Because each wave of air carries away a certain portion of heat [Pg 55] and being followed by another portion of air, a further amount of heat is borne away. <hr class="bible-verse" /> "Though I walk in the valley of the shadow of death I will fear no evil, for thou art with me."—Psalm xxiii. <hr class="bible-verse" /> 176. Is the atmosphere ever as hot as the human body? Not in this country. On the hottest day it is 10 or 12 deg. cooler than the temperature of our bodies. 177. What is the highest degree of artificial heat which man has been known to bear? A man may be surrounded with air raised to the temperature of 300 deg. (the boiling point being 212), and yet not have the heat of his body raised more than two or three degrees above its natural temperature of from 97 deg. to 100 deg. 178. Why may man endure this degree of heat for a short time without injury? Because the skin, and the vessels of fat that lie underneath it, are bad conductors of heat. And because perspiration passing from the skin and evaporating, would bear the heat away as fast as it was received. Because, also, the vital principle (life) exercises a mysterious influence in the preservation of living bodies from physical influences. 179. Is the air ever hot enough, in any part of the world, to destroy life? Yes. The hot winds of the Arabian deserts, which are called simooms, scatter death and desolation in their track, withering trees and shrubs, and burying them under waves of hot sand. When camels see the approach of a simoom they rush to the nearest tree or bush, or to some projecting rock, where they place their heads in an opposite direction to that from which the wind blows, and endeavour to escape its terrible violence. The traveller throws himself on the ground on the lee side of the camel, and screens his head from the fiery blast within the folds of his robe. But frequently both man and beast fall a prey to the terrible simoom. 180. Why are these hot winds so terrible in their effects? Because, being in motion, they search their way to every part of [Pg 56] the body, and passing over it leave some portion of their heat behind, which is again followed by additional heat from every fresh blast of wind. <hr class="bible-verse" /> "The fear of the Lord is the beginning of knowledge: but fools despise wisdom and instruction."—Proverbs i. <hr class="bible-verse" /> <hr class="chap" /> <div style="break-after:column;"></div>

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